#include <stdio.h>
int n,K,nr=0,i;
float P=0,S=0,R,a[50];
void back(int k,int poz,float S){
	if(k==K){
		P+=S;
	nr++;
	return ;
	}
	for(i=poz+1;i<n-K+k+1;i++)
		back(k+1,i,S*a[i]);
}
int main(){
	freopen("dezastru.in","r",stdin);
	freopen("dezastru.out","w",stdout);
	scanf("%d%d",&n,&R);
	for(i=0;i<n;++i)
		scanf("%lf",&a[i]);
	back(0,-1,1);
	P/=nr;
	printf("%.6lf",P);
	fclose(stdout);
	return 0;
}